A light-emitting element using a light-emitting material has advantages of thinness, lightness in weight, fast response, direct-current low-voltage driving, and so on, and is expected to be applied to a next-generation flat panel display. Further, a light-emitting device having light-emitting elements arranged in a matrix pattern is superior to a conventional liquid crystal display device in a wide viewing angle and high visibility.
A light-emitting element is said to have the following light-emission mechanism; voltage is applied to a light-emitting layer sandwiched between a pair of electrodes, electrons injected from a cathode and holes injected from an anode are recombined in a light-emission center of the light-emitting layer to form molecular excitons, and then light is emitted by releasing energy when the molecular exciton returns to the ground state. As the excited state, a singlet-excited state and a triplet-excited state are known, and the light emission is considered possible via either one of the excited states.
In order to enhance the characteristic of such a light-emitting element, the improvement of the element structure, the development of the material, and so on have been conducted.
For example, a method in which an optical length L from a light-emitting portion to a reflective electrode is controlled by sandwiching ITO between the light-emitting portion and the reflective metal is given as means for increasing the external quantum efficiency without deteriorating the luminance by controlling the distance from the light-emitting region to the reflective metal (see, for example, Reference 1: Japanese Patent Application Laid-Open No.: 2003-272855).
An element structure disclosed in Reference 1 is schematically shown in FIG. 2. In this element structure, a transparent electrode 201, a light-emitting portion 202, a transparent conductive film 203, and a metal electrode 204 are stacked. By adjusting the thickness of the transparent conductive film 203, the optical length L from the light-emitting region to the metal electrode is optimized to increase the external quantum efficiency.
However, according to the structure disclosed in Reference 1, since the transparent conductive film 203 and the reflective metal (metal electrode) 204 are in contact, electric erosion might occur due to the difference in their self-potential (see, for example, Reference 2: Japanese Patent Application Laid-Open No.: 2003-89864). Reference 2 describes the self-potential measured using a sodium chloride solution of 3.5% (liquid temperature of 27° C.) and using silver/silver chloride as a reference electrode. Under this condition, the self-potential of aluminum known as reflective metal having high reflectivity is approximately −1550 mV, while that of ITO serving as a transparent conductive film (In2O3-10 wt % SnO2) is approximately −1000 mV. Thus, the difference between these self-potentials is large. Therefore, it is very likely that oxidation-reduction reaction progresses at an interface between aluminum and ITO, which results in electric erosion.
The self-potential is potential of a reaction to a reference electrode when the reaction is soaked in a certain solution in such a state that current is not applied from outside, that is, potential in a closed loop and is also referred to as resting potential.